The present invention relates to settable compositions. More particularly, the present invention relates to settable compositions that comprise magnesium oxide and at least one divalent salt and associated methods of use.
Settable compositions are commonly used in subterranean operations. As used herein, a settable composition refers to a composition that is capable of setting so as to form a hardened mass. Settable compositions may be used in the drilling and completion of well bores to stabilize unconsolidated portions of a subterranean formation, e.g., those comprising clays, shales, sandstones, and the like. Unconsolidated portions of formations include those that contain loose particulates and those wherein the bonded particulates have insufficient bond strength to withstand the forces produced by the production of fluids. Another example of where settable compositions may be used is to seal portions of a formation, such as to prevent the loss of fluid circulation or to prevent the undesirable migration of formation fluids and/or treatment fluids. Settable compositions may also be used in primary cementing operations, inter alia, to support and position a casing string in a well bore, and in reverse and remedial cementing operations. Another use of settable compositions may be to form a “chemical casing” in a well bore, e.g., to stabilize the formation.
Conventional settable compositions may comprise a wide variety of cementitious materials such as hydraulic cement (e.g., Portland cement), fly ash, slag, resins, and the like. Another conventional settable composition may be formed by the mixture of magnesium oxide with aqueous magnesium chloride to form magnesium oxychloride cement, which is commonly known as “Sorel cement.” A similar settable composition may be formed by the mixture of magnesium oxide with aqueous magnesium sulfate. Settable compositions comprising a cementitious material, such as those described above, may be placed into a desired location within a subterranean formation and allowed to set to form a hardened mass. Generally, these settable compositions may be placed into the desired location as one fluid system, which can be problematic. For example, problems may occur when the settable composition begins to harden before it has reached its desired location within the subterranean formation. Additionally, use of these conventional settable compositions may be problematic due to placement difficulty, cost, reactivity, local availability, and compatibility with both formation and drilling fluids.